How It Works: Linear Resonant Actuators

Diagram of the construction of a linear resonant actuator.

A linear resonant actuator is a vibration motor that produces an oscillating force across a single axis. Unlike a DC eccentric rotating mass (ERM) motor, a linear resonant actuator relies on an AC voltage to drive a voice coil pressed against a moving mass connected to a spring. When the voice coil is driven at the resonant frequency of the spring, the entire actuator vibrates with a perceptible force. Although the frequency and amplitude of a linear resonant actuator may be adjusted by changing the AC input, the actuator must be driven at its resonant frequency to generate a meaningful amount of force for a large current.

The voice coil remains stationary inside of the device while it produces a vibration and presses against a moving mass. By driving the magnetic mass up and down against the spring, the LRA as a whole will be displaced and produce a vibration. The underlying mechanism resembles a speaker producing sound. In a speaker, air is funneled through a cone and displaced at different frequencies by turning an AC frequency and amplitude into a vibrational frequency and amplitude—internally, a speaker accomplishes this task by moving a magnetic mass with a fast-changing alternating current. Unlike a speaker, which can be driven at arbitrary frequencies, an LRA is useful in haptic applications within a specific frequency range.

Rather than directly transferring the force produced by the voice coil to the skin, the device optimizes for power consumption by taking advantage of the resonant frequency of the spring. If the voice coil pushes the magnetic mass against the spring at the spring’s resonant frequency, the device can produce a vibration of higher amplitude more efficiently. Since the voice coil is driven by an AC current, modeling the desired frequency and amplitude of vibration, the frequency and amplitude may be independently modified. This is different than an ERM motor, which couples the two properties of the resulting vibration.

Although the frequency can be changed, the LRA will typically be operated within a narrow frequency range to optimize its power consumption—if the device is driven at the resonant frequency of the spring, it will consume less power to produce a vibration of equal magnitude. Regardless, this improvement alone presents a unique advantage over ERM motors: a precise waveform of varying intensity over time can be reproduced in an LRA with a fixed frequency, whereas a waveform of varying intensity in an ERM motor will also produce a varying frequency of vibration.

The typical start time for an LRA is approximately 5-10ms, a fraction of the time required to produce a vibration with an ERM motor. This incredible speed results from the immediate movement of the magnetic mass as current is applied to the voice coil inside of the device. In an ERM motor, the vibration can only be produced after the motor reaches its operating speed—even when overdriving the motor to produce faster acceleration, the motor can require 20-50ms before reaching a desired intensity of vibration. Unfortunately, the stop time of an LRA can be significantly longer than an ERM motor. An LRA can take up to 300ms to stop vibrating due to the continued storage of kinetic energy in the internal spring during operation. Thankfully, an active braking mechanism can also be used for an LRA—by performing an 180-degree phase shift of the AC signal provided to the actuator, the vibration can be stopped very quickly (within approximately 10ms) by producing a force opposite to the oscillation of the spring.

Consumer Technology

The video below shows a teardown of the iPhone 4S, which used a linear resonant actuator attached to the device enclosure to provide vibrotactile feedback.

Many modern cell phones use an LRA instead of other types of vibration motors in order to produce a wider range of vibrotactile effects with less power. The Steam video game controller from Valve also makes use of linear resonant actuators to provide trackpads with haptic feedback.

Prototyping

The DRV2605 Breakout Board from Adafruit Industries.

Adafruit Industries sells the DRV2605 Breakout Board with headers suitable for a breadboard prototype of a device using a linear resonant actuator. The aforementioned breakout board can be easily integrated with an Arduino, Raspberry Pi, Beaglebone, or other electronics prototyping platform. You can also find other variations of the breakout board online, including the compact DRV2605 modules from Fyber Labs.

Manufacturers

The supply chain for linear resonant actuators is not as robust as the availability of ERM motors. Regardless, there are a few sources for the component: